1. Field of the Invention
The present invention relates to an apparatus for drying a moist particulate material having a non-uniform particle size with superheated steam, the apparatus including a lower cylindrical part having a number of parallel, substantially vertical elongated chambers located in ring form, one or more of the chambers having a closed bottom and the remaining chambers having a steam-permeable bottom, the adjacent chambers being interconnected through openings in the chamber walls at the lower ends of the chambers, and an upper conical part which is also divided into chambers, which at its lower end is connected to the chambers of the cylindrical part of the apparatus and which at the top is connected to a transfer zone, the chambers of the conical part of the apparatus being divided into smaller chambers by means of inclined guide plates, means for supplying moist particulate material to a chamber having a steam-permeable bottom, means for discharging dried material from a chamber having a closed bottom, means for supplying superheated steam to the area below the steam- permeable chamber bottoms, means for discharging steam from the transfer zone, and means for reheating the discharged steam and recirculating it to the area below the steam-permeable chamber bottoms.
2. The Prior Art
An apparatus of the type mentioned above is known from Zuckerind. 114 (1989) No. 12, pp. 964-70 and EP-A-0153704. This prior art apparatus is particularly suitable for drying beet pulp formed by extracting sugar from sugar beet slices with water, but the apparatus is also suitable for removing liquid, including liquids other than water, from a number of sensitive organic materials.
The prior art apparatus presents the advantage that the particulate material is dried without the access of air, thereby making it possible to avoid oxidation of the material during drying. Another important advantage of the apparatus is that it is environmentally highly acceptable as the drying takes place in a substantially closed system. Furthermore, the excess amount of steam, which, e.g., is generated when drying beet pulp, is very pure and consequently it can be used for the concentration of sugar juice, and the condensate thus formed does not cause odour nuisances as compared to the emission products formed by, e.g., drum-drying beet pulp.
In practical use of the drying apparatus mentioned above it has been found that partially dried particulate material tends to adhere to the oblique guide panels and in particular to the upper sides of these panels and that a gradually increasing coating is formed on the guide plates.